Expensive. That's basically it. We are using Bitbucket for that reason. We aren't a large team, but we do have quite a few repos. On Github we'd be paying $50-100 per month, on Bitbucket we pay nothing. Not a big deal for a heavily funded company, but we're bootstrapping for the most part, so it's a big deal for us.

Of course that might not be a sustainable state of affairs in a big picture sense, but it works for us for right now.

I had to use this feature at a former job. I have a Github account with several open-source projects running, so the mixing of public and private repositories in the same account was annoying. I could've created a different account but then I'd have to juggle two different accounts all the time. Oh, and this private repository was on another account, so I think I wouldn't have been able to fork it onto my account without "forking out" some cash for Github, because private repos cost money. So the triangular workflow with pull requests would've been out of the question.